The long-range goal for most of the experiments is to provide a functionally-relevant description of the patterns of connectivity of those portions of the rostral brainstem in mammals that are involved in somatic sensation. A group of experiments on cats is proposed for this budget period whose aim is to specify anatomically the sorting processes that occur as fibers from the spinal cord and the dorsal column, trigeminal and lateral cervical nuclei diverge and reassemble upon entering the diencephalon. Such specification requires knowledge of which neurons within each pathway project to the different parts of the brainstem. In the first group, at least three discriminable and compatible retrograde markers (e.g. HRP, materials which fluoresce, 3H-apoHRP) will be injected in the same cats into different terminal targets of the regions listed above. The projection patterns of tne neurons in these regions will be determined from the resultant retrograde marking patterns. These targets include: ventrobasal complex (VB), its border areas, posterior group complex, magnocellular medial geniculate n., zona incerta, intralaminar group, pretectal area, inferior olive, cerebellum and spinal cord. In related experiments designed to examine the microhodology of VB, retrograde marking patterns will be examined after very small injections of the markers into restricted portions of VB. Two additional collaborative studies, unrelated to the main goal, are also proposed. In the first, the efferent projections of the suprachiasmatic n. of the rat hypothalamus will be studied using autoradiographic techniques. In the second, autoradiographic degeneration and HRP techniques will be used to determine if the projections from the somatic sensory cortex and/or the dorsolateral funiculus to the dorsal column nuclei are reorganized as a result of long-term ablations of the dorsal columns in monkeys.